This invention relates to connectors for connecting an integrated circuit chip carrier to a printed circuit board and, more particularly, a connector which provides a separable assembly and a relatively flat overall package.
When packaging an electronic component for a pocket or laptop computer, a requirement that must be strictly adhered to is that the height of the component when mounted on a printed circuit board must be kept to a minimum. Integrated circuit chips are conventionally carried on modules which are like miniature printed circuit boards on which an interconnection pattern and contact pads are deposited. For use in lap-top and pocket computers there is presently being developed a new integrated circuit package which consists of a thin printed circuit board having an array of plated through holes. The bottom side of the circuit board has an array of contact pads, each connected to a corresponding plated through hole. Each contact pad has an electrical connection through the hole to the top side of the circuit board, which is typically very thin, permitting the diameter of the plated through holes to be very small, for example on the order of 0.004 inches. On the top side of the circuit board, there is an area designated for mounting the integrated circuit chip, surrounded by pads, to which wire bonding or tape automated bonding, or the like, from the chip can be made. These pads are typically arranged in a rectangular pattern, corresponding to the perimeter array of bonding pads on the integrated circuit chip. From these pads, conductors extend, each to a corresponding plated through hole on the circuit board. Accordingly, each of the chip pads is electrically connected to a corresponding contact pad on the bottom of the printed circuit board.
After the integrated circuit chip is attached to the printed circuit board, the board is inserted into a mold and plastic is injected, encapsulating the chip and a substantial portion of the area of the printed circuit board on the surface carrying the chip. In the area outside the chip, the injected plastic enters the plated through holes. Since injection molded plastic has no adhesive-like bonding characteristic to the material over which it is injected, the mechanical anchoring is attained by the plastic entering the holes, which by virtue of the manufacturing process have substantially uniformly distributed surface irregularities and a substantial anchoring force is thereby achieved.
In one version of this package, the bottom side of the printed circuit board which is populated with contact pads, is further populated with solder balls. This subassembly is then placed on top of a larger circuit board, which has corresponding contact pads and the part is reflowed or soldered in place. This method of attachment of a component to a circuit board has two major disadvantages. A first disadvantage is that due to the mismatch of the coefficients of expansion, the size of the package is limited to very small dimensions, such as for example 1/4 inch square. Larger parts attached in this manner will suffer fracture of the solder connections. A second disadvantage is that there are many circumstances when in addition to requiring larger packages and larger numbers of leads, a separable package is required. In addition to the requirement of separability, which can be met by assemblies such as those disclosed in U.S. Pat. No. 4,699,593, there are additional requirements of densities at which the assembly disclosed in the referenced patent becomes impractical, and the height of such an assembly is excessive, especially in the case of satisfying the requirements for laptop or pocket computers.
It is therefore a primary object of the present invention to provide a separable connector assembly which achieves high density connection of an integrated circuit chip module to a printed circuit board.
It is a further object of this invention to provide such an assembly in a flattened structure which satisfies the requirements of laptop or pocket computers.